Acrylic structural adhesives are well known articles of commerce which are extensively used commercially for bonding metal and plastic materials. Acrylic structural adhesives have found growing use in the automotive industry where the adhesive bonding of metal parts is replacing welding and mechanical fastening techniques. However, these applications give rise to unique requirements not easily met by previously available adhesives. These requirements include high bond strength and improved failure mode.
Acrylic adhesives typically comprise a mixture of one or more olefinic reactive monomers such as methyl methacrylate or methacrylic acid and curing agents, with cure or polymerization being effected through a free radical polymerization mechanism. The adhesives preferably contain one or more polymeric materials which may or may not be reactive, that is, capable of being polymerized per se or at least capable of interpolymerizing with the reactive monomers, such as grafting onto or crosslinking the growing polymers from the reactive monomer polymerization. In addition, the adhesives can contain other additives for improving adhesion to substrate materials, environmental resistance, impact strength, flexibility, heat resistance, and the like.
P.C.T. Publication WO 97/39074 discloses the use of at least one polymeric material which can act as a toughening agent in an acrylic adhesive such as polychloroprene, polymer-in-monomer syrup, chlorosulphonated polyethylene rubber, copolymers of butadiene and at least one monomer copolymerizable therewith, for example, styrene, acrylonitrile, methacrylonitrile (e.g. poly(butadiene-(meth)acrylonitrile or poly(butadiene-(meth)acrylonitrile-styrene) and mixtures thereof; as well as modified elastomeric polymeric materials, such as butadiene homopolymers and copolymers as noted above modified by copolymerization therewith of trace amounts of up to about 5 percent by weight of the elastomeric material of at least one functional monomer (such as acrylic acid, methacrylic acid, maleic anhydride, fumaric acid, styrene, and methyl methacrylate to give, for example, methacrylate-terminated polybutadiene homopolymers and/or copolymers). Also disclosed is a redox catalyst system that includes certain para-halogenated aniline reducing agents.
U.S. Pat. No. 4,769,419 ("the '419 patent"), incorporated herein by reference, discloses structural adhesive compositions for metal-metal bonding applications. The adhesives include as tougheners olefinic terminated liquid rubbers which are reacted with monoisocyanate compounds. These tougheners represented by formula I as shown in column 4 and claim 1 of the '419 patent are referred to herein as "urethane modified olefinic-terminated liquid elastomer produced from a carboxylic acid-terminated alkadiene" or, alternatively, as "isocyanate-capped methacrylate-terminated polyalkadiene produced from a carboxylic acid-terminated polyalkadiene". The '419 patent further describes acrylic adhesives with olefinic monomer, olefinic urethane reaction product of an isocyanate-functional prepolymer and a hydroxy-functional monomer, phosphorus-containing compounds, an oxidizing agent and a free radical source.
U.S. Pat. No. 5,641,834 ("the '834 patent") and U.S. Pat. No. 5,710,235, both incorporated herein by reference, disclose adhesives that include as tougheners an olefinic-terminated polyalkadiene that includes carboxy ester linking groups and at least one nascent secondary hydroxyl group that is capped with a monoisocyanate. These tougheners represented by the formulae for polymer A or polymer B in claim 1 are referred to herein as "olefinic-terminated liquid elastomer produced from a hydroxyl-terminated polyalkadiene" or, alternatively, as "isocyanate-capped methacrylate-terminated polyalkadiene produced from a hydroxyl-terminated polyalkadiene". The composition also includes a free radical-polymerizable monomer such as an olefinic monomer and, optionally, a second polymeric material. In a preferred embodiment the composition is an adhesive that also includes a phosphorus-containing compound and an ambient temperature-active redox catalyst. The '834 patent describes polymers A and B with a number average molecular weight of about 1,500 to 10,000, preferably about 1,500 to 6,000.
Peel strength and failure mode of the above-described prior structural adhesives leave room for improvement.
U.S. Pat. No. 5,728,759 ("the '759 patent") discloses non-structural waterbased pressure sensitive adhesives and methods of preparation including an adhesive entity, a tackifier resin, a plasticizer, stabilizer, curing entity and optional additives. Objects of the '759 patent include providing masking tapes that do not freeze to an automobile body or window on exposure of the tapes to relatively high temperatures while in contact with such surfaces. Elastomeric block copolymers usable in the adhesive entity include block copolymers "EUROPRENE.TM. Sol T 193" from Enichem AMERICAS.
U.S. Pat. No. 5,500,293 is directed to a non-structural adhesive composition suitable for use in an insulating tape having improved plasticizer resistance, including from about 13% to about 42% of a polyisoprene homopolymer, from about 13% to about 42% of a styrene-isoprene-styrene copolymer, and from about 25% to about 55% of an aliphatic tackifying agent. The styrene-isoprene-styrene copolymers disclosed include EUROPRENE.TM..
Additional important features of acrylic adhesives are surface tack and open time.
As used herein, "surface tack" means the amount of adhesive on an exposed surface of the applied adhesive that does not undergo curing. Such uncured adhesive can be transferred to other parts of the assembly or to the application equipment resulting in increasing clean-up costs. A common cause of surface tack is referred to as "air inhibition" since atmospheric oxygen is a powerful inhibitor of free radical reactions. Accordingly, the amount of surface tack can be measured by determining the thickness of any uncured adhesive on the surface.
In typical applications of two-part reactive adhesive systems, the two parts are mixed together, the mixed material is applied to a first substrate for bonding then a second substrate is contacted to the adhesive-applied first substrate. The time required for such mixing, applying and contacting is referred to herein as "open time". Of course, the substrates must be contacted together prior to final cure of the adhesive.
Although the surface tack and open time of acrylic adhesives commercially available from Lord Corporation are more than adequate for many applications these characteristics could be improved.